Midsoles of sports shoes generally require a few important properties, such as good resilience, appropriate softness and lightweight, apart from having a certain level of hardness.
In order to obtain the aforementioned properties, midsoles for shoes are usually made from a foamable plastic composition including ethylene vinyl acetate (EVA). By foaming the foamable plastic composition including EVA, midsoles can achieve a certain level of hardness while possessing good resilience, appropriate softness and lightweight. Although conventional plastic compositions including EVA provide midsoles having appropriate softness and good resilience not less than 55%, the shrinkage rate thereof is undesirably greater than 1.5%, and is unable to satisfy the industrial standards of footwear manufacturing. From the conventional plastic compositions, it is hard to produce midsoles having a high dimensional accuracy. U.S. Pat. No. 8,461,222 B2 discloses a method of making a foamable material for producing midsoles, which is made from a composition including thermoplastic polyolefins. However, the resilience of the midsoles produced therefrom is lower than 55% and cannot meet the high-resilience requirement for midsoles of sport shoes.
Furthermore, the compositions in the prior art encounter another problem. When different midsoles having varying properties, such as varying resilience and shrinkage rate, are to be produced, it is necessary to select different compositions and different foaming ratios according to the requirements of the midsoles. Even if midsoles are produced from the same composition, it remains necessary to have different foaming ratios. Therefore, the cost of manufacturing midsoles will increase since different forming molds are needed for different foaming ratios.